My brother-in-law has a theory about why he and so many of his colleagues at work have daughters rather than sons. He believes that spending 12 hours in front of a computer screen makes him unable to pass on his Y chromosome, and hence unable to produce male progeny.

This idea may seem outlandish, but it is no more far-fetched than some widely accepted gender-selection theories throughout history, such as the influence of the weather and the temperature of the mother’s womb.

These and similar nuggets of sexual history are discussed in two new books about the sex chromosomes, which come in two flavors, X and Y. Women have two X chromosomes, while men have one of each.

In The X in Sex: How the X Chromosome Controls Our Lives, author David Bainbridge explores the history, the science, the philosophy, and the many theories that surround the mighty X chromosome, which he calls “the most compelling little scrap of stuff in existence.”

In this slim, interesting and easy-to-read volume, Bainbridge, a lecturer in comparative anatomy and physiology at the Royal Veterinary College in London, encapsulates our knowledge of the genes that determine one’s sex. He explores historic speculations about the origins of gender and surveys the recent scientific literature, telling us what some of the genes on these chromosomes actually do.

As Bainbridge asserts, it has been a long road from Gregor Mendel’s seminal work on heredity in the 19th century to our current understanding of sexual determination. We now know that the Y chromosome, which was sequenced in 2003, is significantly smaller in size than the X and that it seems to be a “single-issue” chromosome designed to determine sex.

The X, on the other hand, controls our lives in thousands of ways, though it was once thought to be merely a passive counterpart of the Y. The nature and evolution of these chromosomes have had very different consequences for men and women.

For males, there is “the curse of the lone X.” They have ended up with an array of inherited conditions and diseases related to problems on the X chromosome. These range in nature from baldness and color blindness to muscular dystrophy and hemophilia.

Indeed, males generally suffer for having just one copy of each gene. When a gene on the X is damaged, Bainbridge notes that “all hell breaks loose” because there is no spare copy to fall back on.

While males struggle with their lone X, females deal with having two copies of the same chromosome by “turning off” one copy of each gene in every cell. Thus, females in effect have one functional X chromosome, the same as men.

Females have sex-specific diseases and conditions as well. Some seem mundane, such as the fact that there are more female than male identical twins, while others more serious. Women are disproportionately affected by diseases of the immune system, though the reasons are not clear.

After finishing David Bainbridge’s fascinating introduction to the X chromosome, I read Steve Jones’s Y: The Descent of Men.

Jones is a professor of genetics at University College in London, and the title is a play on the Darwin classic on evolution, The Descent of Man. The “descent” in Jones’s title refers to the ostensible thesis of his book: that the male sex, and maleness itself, is in decline.

The Y chromosome, he argues, is a symbol of a biological and sociological loss of potency.

According to Jones, “Males are wilting away. From sperm count to social status, and from fertilization to death, as civilization advances those who bear the Y chromosome are in relative decline.”

Jones built his reputation on updating the work of Charles Darwin and making it more accessible to the public. His previous book was the much-heralded Darwin’s Ghost, in which he followed the structure of The Origin of Species and gave it a modern perspective.

But in Y, he presents no convincing biological evidence to support the idea that this so-called decline in male power has anything to do with DNA. His arguments point instead to social reasons, like the rise of feminism, as the cause for this descent.

While the book is filled with interesting details, its premise is misleading, and the book is, ultimately, unsatisfying. Jones discusses the recent alarming rise in the rate of male suicides, for instance, which has doubled in the last 30 years while the rate for women remained steady. But what does this have to do with the shrinking of the Y chromosome?

Y is a hodge-podge of ideas about genetics, the history of male sexual behavior, and more information than anyone would ever need on the mechanics, aberrations, and abuse of the male genital organs. (There are 11 pages on circumcision alone, followed by disturbing material on castration practices.)

There is a great deal of titillating but often unnecessary detail accompanied by unsupported commentary. In a book about the Y chromosome, more genetics would have been welcome.

Having read Bainbridge’s brilliant introduction to the X chromosome, I was looking forward to an equally rewarding read in Steve Jones’s Y. At the very least, it would have been good to learn more about how the presence of the Y chromosome creates a difference between male and female genders.

Instead, Jones bases his case about the decline of maleness largely on sociology. Perhaps this job should have been left to sociologists, who might have handled the subject more deftly than he.